Electron discharge tube



Feb. 2, 1937. N. M. RUST El AL 2,059,507

ELECTRON DISCHARGE TUBE I Filed J1me 29,- 1935 F'gui fig 2 Patented Feb. 2, 1937 UNITED STATES PATENT OFFICE ELECTRON DISCHARGE TUBE Delaware Application June 29, 1933, Serial No. 678,128 In Great Britain July 4, 1932 5 Claims.

This invention relates to electron discharge amplified tubes or valves and has for its main object to provide a thermionic tube or valve wherein unwanted self-capacity effects between the input and output electrodes are greatly reduced or substantially eliminated.

The so-called screened grid type of tube is a well-known construction wherein such unwanted inherent capacity effects are largely reduced by providing an electrostatic screen between the anode and the control grid of the tube but it is found that even with this construction there is some inherent or self-capacity between the anode and control grid, and the present invention provides a tube construction wherein such self-capacity may be reduced even below that obtained in a screened grid tube.

According to this invention the improved tube comprises an anode, a cathode, a. control electrode placed relatively closely to said cathode and a screen or shield electrode placed relatively closely to the last mentioned two electrodes to shield the control electrode from the anode, the shield electrode being so arranged as to permit only a narrow beam or stream of electrons to pass the said shield electrode and reach the anode.

The invention is illustrated in the accompanying drawing in which Figs. 1 and 2 are schematic longitudinal sectional elevations in perpendicular planes of one form of tube constructed in accordance with the invention; Fig. 3 is a schematic plan view looking down along the line IIIIII of Fig. 1, of only the triode assembly of the electrodes, the glass container being omitted, Fig. 4 is a chematic' longitudinal sectional elevation of a modification, and Fig. 5 is a schematic longitudinal sectional elevation of still another modification.

Referring to Figures 1 to 3 the thermionic tube E comprises an electron collecting anode P, a cathode F in the form of a linear filament, and a control electrode. G in the form of a tubular electrode surrounding the linear filament. An accelerating anode A which also acts as a screen or shield electrode is placed in front of the structure including the filament F and the control electrode, this accelerating anode or screen electrode being in-the form of a fiat plate having a hole or opening Al and placed a short distance away from the open end of the control electrode and, of course, on the side thereof towards the collecting anode P. The collecting anode P is mounted in the other end of the tube envelope and coaxial with respect to the accelerating elec trode A so that the whole arrangement is such that an imaginary line from the mid-point of the filament passing through the centre of the aperture in the accelerating anode and in a direction at right angles to that electrode will pass 5 through the centre of the collecting anode. A fixed positive potential is applied to the accelerating anode, and control or signal potentials are applied to the electrode G which acts both as an electrostatic focussing electrode and as a control electrode. It is found that if the electrodes be suitably placed and suitable potentials be applied thereto a. concentrated beam of electrons of several milliamperes in value can be obtained and collected at the collecting anode. It is not necessary to provide any gas filling for focussing purposes since this may be accomplished purely electrostatically by the control electrode. The accelerating or screen electrode A is, as above set out, at a fixed high potential and if the aperture therein is of suitable size the current reaching the said accelerating electrode will be very small or negligible compared with that arriving at the collecting anode. It has also been found that with this construction the number of electrons reaching the collecting anode will vary quite considerably over a certain range of negative potentials applied to the control electrode in dependence upon those potentials, the shape of the electron beam remaining practically un- 30 altered. Owing to the fixed potential applied to the accelerating or screen anode A the collecting anode P is substantially entirely shielded from the control electrode G and owing to the concentration of the beam the separation between the said collecting anode and the said control electrode G can be made relatively large, and the size of the said anode relatively small, thus further reducing capacity eifects. The electrode G and collecting anode P may therefore be regarded as a low capacity substitute for the control grid and anode of an ordinary normal type of thermionic tube.

In practice the collecting anode is maintained at approximately the same potential as the ac- 45 celerating electrode and hence there exists no large back electromotive force to prevent the escape of secondary electrons from the collecting anode. One way of avoiding this dimculty is to provide just in front of the collecting anode a 50 supplementary electrode maintained at a suitable potential to provide a retarding field near the collecting anode. For example, in the case in which the collecting anode P is a simple plate, as illustrated in Fig. 4, the supplementary electrode 55 may be a grid electrode S just in front of the collecting anode P and maintained at approximately cathode potential. This electrode S will still further serve to screen the control electrode from the collecting anode.

Another way as shown in Fig. 5 is to make the supplementary electrode S in front of the accelerating anode in the form of a plate-like electrode having an aperture 0 corresponding to that in the said accelerating anode and maintained at zero or some low value of potential so as to provide a retarding field at the collecting anode. A third way is to specially shape the collecting anode. In view of the concentrated nature of the electron stream the collecting anode may be made in the form of an almost closed box having an aperture or slit-like opening in the wall facing the cathode, this aperture or slit being large enough to admit the electron stream. With this construction of collecting anode the interior thereof will be so effectively screened from the accelerating anode or screen that only a small or negligible proportion of secondary electrons can escape. This box-like construction may take any of a variety of different shapes but from the point of view of obtaining the lowestinterelectrode capacity as well as minimizing the effects of small stray fields from the accelerating anode a preferred form is, as illustrated in Figures 1 to 3, that of a long fiat approximately elliptical tubular structure generally resembling the control electrode and having the end remote therefrom closed, the said tubular structure being, of course, approximately coaxial with the said control electrode. In order to still further minimize the escape of secondary electrons, the open end of the tubular collecting anode may be covered or closed by an open meshed grid or net N.

It will be seen that the arrangement just described gives what may be termed a sheet of electrons and is thus suitable for use in cases where large currents of high concentration are desired. Where large currents are not required a jet of electrons may be substituted for the sheet, i. e., the electron stream may approximate in cross section to a point instead of to a line. In order to obtain this result it is necessary to modify the previously described construction by substituting for the linear cathode providing a source of electrons approximating to a line, a cathode or filament of hairpin form providing a source of electrons approximating to a point; substituting for the control electrode of flattened form with its longer dimension parallel to the line of the filament a simple cylindrical control electrode; providing an ordinary round aperture instead of a slit in the accelerating anode and shaping the anode and/or other electrodes (if any) to adapt them to cooperate with the jet-like beam of electrons.

For most purposes the construction in accordance with this invention generally to be preferred is as shown in Figures 1 to 3 and comprises an evacuated envelope containing at one end a flat tubular collecting anode as illustrated in Figures 1, 2, and 3, having its axis lying along the axis of the envelope, this anode being closed at the end remote from the cathode and open (or closed with a grid mesh) at its other end. The connection to the collecting anode is taken through the adjacent end of the envelope. Mounted in the other, or stem end of the envelope is an electrode system consisting of a linear filament extending at right angles to the axis of the envelope, a control electrode of flattened form surrounding said filament and an accelerating electrode just in front of the said control electrode, said accelerating electrode having a slit-like aperture parallel to and corresponding to the length of the filament, and to the longer of those two dimensions of the control electrode which are at right angles to the axis of the envelope, and to the longer of those two dimensions of the collecting anode which are at right angles to the axis of the envelope. The connections to the filament F, control electrode G, and to the accelerating electrode A are brought out through the stem which is accordingly provided with a four wire press.

The dimensions of the electrodes are not critical but the following have given good results:

Filament; 5 mm. long.

Accelerating anode; a rectangular plate 10 mm. by 3 mm. having an aperture of from 3 to 5 mm. long and 1 mm. wide.

Collecting anode; a flattened cylinder or tube of rectangular section having one dimension of the section of from 5 to 10 mm. and the other of from 2 to 3 mm. the length of the tube being from 10 to 15 mm.

The following results were obtained:

Potential applied to collecting and accelerating anodes: -300 volts.

Potential applied to control electrode: 10-30 volts negative.

Current at collecting anode: 3-5 milliamperes.

Change in current at collecting anode per volt change on control electrode: 0.2-0.3 milliamperes.

Having now particularly described and ascertained the nature of our invention and in what manner the same is to be performed, we declare that what we claim is:-

1. An electron discharge tube comprising an evacuated vessel, a cathode disposed in a plane at right angles to the vertical axis of said vessel, a ring-shaped control electrode surrounding said cathode, a perforated disc-shaped electrode parallelly arranged with respect to the cathode and closely positioned with respect to the control electrode, and a tubular anode electrode positioned on that side of the perforated electrode remote from the cathode, said anode having an opening only at the end nearest the cathode, and a screen mesh material enclosing said open end.

2. An electron discharge tube comprising an evacuated vessel, a cathode disposed in a plane at right angles to the vertical axis of said vessel, a ring-shaped control electrode surrounding said cathode, a perforated disc-shaped electrode parallelly arranged with respect to the cathode and closely positioned with respect to the control electrode, and a tubular anode electrode positioned on that side of the perforated electrode remote from the cathode, said anode having an opening only at the end nearest the cathode, and a ring-shaped electrode closely adjacent the open end of said anode and electrically connected to the cathode.

3. An electron discharge tube comprising an evacuated vessel, a cathode disposed in a plane at right angles to the vertical axis of said vessel, a ring-shaped control electrode surrounding said cathode, a perforated disc-shaped electrode parallely arranged with respect to the cathode and closely positioned with respect to the control electrode, and a tubular anode electrode positioned on that side of the perforated electrode remote from the cathode, said anode having an opening only at the end nearest the cathode, and a ring-shaped electrode provided with an opening of similar shape as that of the anode, closely adjacent the open end of the anode and electrically connected to the cathode.

4. An electron discharge tube comprising a linear electron emitting cathode transversely arranged with respect to the tube axis, a flattened tubular anode electrode extending longitudinally within thetube and with its major axis in the transverse plane parallel to the cathode, a similarly-shaped control electrode, for controlling the electron stream between said first mentioned electrodes in response to signal currents, surrounding the cathode, and a disc-shaped electron accelerating electrode parallel to the cathode interposed between the control electrode and the anode electrode and provided with a slit-like aperture which extends in a direction parallel to the cathode.

5. An electron discharge tube comprising coaxially-arranged tubular control and anode electrodes, a cathode positioned within the control electrode, a screen electrode interposed in the space between the control and anode electrodes but closer to the former, said screen electrode having a slit-like opening to permit only a narrow stream of electrons to pass from cathode to anode, and a screen mesh material enclosing that end of the tubular anode nearest the cathode.

NOEL MEYER RUsT. GEORGE FAIRBU'RN BRETT. 

